In a semiconductor device fabrication process, a wafer having a device area with a plurality of devices, commonly partitioned by a plurality of division lines, is divided into individual dies. This fabrication process generally comprises a grinding step for adjusting the wafer thickness and a cutting step of cutting the wafer along the division lines to obtain the individual dies. The grinding step is performed from a back side of the wafer which is opposite to a wafer front side on which the device area is formed. Moreover, also other processing steps, such as polishing and/or etching, may be carried out on the back side of the wafer. The wafer may be cut along the division lines from its front side or its back side.
In order to protect the wafer back side or front side, in particular, the devices formed in the device area, e.g., from breakage, deformation and/or contamination by debris, grinding water or cutting water, during processing of the wafer, a protective film or sheeting may be applied to the back side or front side of the wafer prior to processing.
Such protection of the wafer back side or front side, in particular, the devices, is especially important if the respective side of the wafer, e.g., the device area, has an uneven surface structure.
For example, in known semiconductor device fabrication processes, such as Wafer Level Chip Scale Package (WLCSP), the device area of the wafer is formed with a plurality of protrusions, such as bumps, protruding from a plane surface of the wafer. These protrusions are used, for example, for establishing an electrical contact with the devices in the individual dies, e.g., when incorporating the dies in electronic equipment, such as mobile phones and personal computers.
Further, in known wafers, the devices in the device area may protrude from a plane wafer surface. The division lines partitioning these devices may form at least slight grooves or trenches between the devices, thus resulting in an uneven surface profile.
In a conventionally known dicing before grinding process, a wafer is first partially cut along the division lines from its front side along only a part of the thickness of the wafer. Subsequently, the wafer back side is ground along a remaining part of the wafer thickness in which the wafer had not been cut, so as to divide the wafer along the division lines in separate chips or dies. Also such partial cuts introduce a surface profile which renders the wafer front side uneven.
When attaching a protective film or sheeting to such an uneven wafer surface in a conventional manner, problems arise in that, e.g., due to the presence of grooves, trenches or protrusions, the surface is not efficiently protected. In particular, the protective film may not reliably seal the wafer surface at a peripheral wafer portion, allowing contaminants, such as debris, grinding water or cutting water, to enter and contaminate the surface.
In order to achieve a size reduction of electronic equipment, such as mobile phones and personal computers, semiconductor devices have to be reduced in size. Hence, wafers having the devices formed thereon are ground in the grinding step referred to above to thicknesses in the μm range, e.g., in the range from 20 to 100 μm. Desirably, such grinding processes are performed at high speed, so as to improve the wafer processing efficiency.
In known semiconductor device fabrication processes, problems may arise during the process of grinding wafers to such small thicknesses, in particular, at high speed. Specifically, the grinding process may cause the formation of sharp wafer edges which act as starting points for wafer chipping and breakage, thus significantly affecting the quality of the resulting chips or dies. Further, equipment used for dividing the wafer after grinding, such as a blade or a saw, may be damaged due to being impacted by loose, small-sized chips arranged at the peripheral wafer portion. Such sharp wafer edges may also lead to problems in the storage and/or transport of ground wafers, e.g., damaging the containers in which they are received, such as wafer cassettes or the like.
In order to address the above-identified problems related to thin wafer grinding, it is known to perform an edge trimming process on the wafer front side prior to grinding the wafer. In this process, at least a part of the outer peripheral portion of the front side of the wafer is cut, so as to obtain a step portion, e.g., an annular step portion, along the wafer circumference. By providing such a step portion, the formation of a sharp wafer edge in the grinding step can be avoided. However, the requirement of an additional edge trimming step renders the wafer processing method more complicated and reduces the processing efficiency.
Hence, there remains a need for an efficient and reliable method of processing a wafer having a device area which allows for any risk of contamination and damage to the wafer to be minimised.